What Is Kraft Liner? Its Role in Carton and Box Strength

Reviewed and updated by ICD - Week 44/2026

Kraft liner (kraftliner) is a type of Kraft paper used as the flat facing layers, outermost and innermost, of corrugated board, made mainly from virgin Kraft pulp. By the definition commonly used in the industry, kraftliner contains at least 80% virgin Kraft fibre, while testliner (recycled liner) contains less; kraftliner grammage on the market is about 115 to 440 g/m². The liner decides most of a carton’s bursting strength, compression strength and printing surface.

I am Le Van Thang, and I have worked in packaging supplies at ICD Vietnam since 2011. Cartons are what dunnage bags press against every day inside a container, so the quality of the liner directly affects whether the load keeps its shape for the whole journey. This article explains what a liner is, the technical figures to read, how to choose a liner, and why liner paper is different from the paper used for dunnage bag shells.

Key takeaways

• Kraftliner is the facing layer of corrugated board, made mainly from virgin Kraft pulp; testliner is liner with a higher share of recycled fibre.
• Important figures: bursting strength, short-span compression in the cross direction (SCT-CD), Cobb water absorption, grammage.
• A good liner raises edge crush (ECT) and box compression strength (BCT), meaning the box can be stacked higher.
• The shell of a Kraft paper dunnage bag uses Kraft paper laminated with thin plastic film, much thinner than liner paper, and is not liner.

1. What is Kraft liner? Its role in carton structure

Kraft liner is a flat paper layer with a smooth surface, used as the outer and inner face of corrugated board; in 5-ply board there is also a middle liner between the two flutes. It is the material that decides most of the load-bearing properties of the packaging.

1.1. Position and function of the liner and the fluting

Single-wall corrugated board has three paper layers: two liners sandwiching a fluted layer (fluting, also called medium) in the middle. The liner takes impact, abrasion and moisture directly and gives the printing surface; the fluting keeps the two liners apart, making the board much stiffer than the sum of its flat layers, and also gives a cushion against shock.

When boxes are stacked, compression travels vertically along the box edges; the liner and the fluting both carry this load. So the quality of both layers together decides box compression strength, and neither layer can replace the other.

1.2. Technical characteristics of Kraft liner

Kraftliner grammage on the market is about 115 to 440 g/m², and shipping packaging commonly uses about 115 to 200 g/m², with heavier grades for industrial boxes (according to PaperIndex). Higher grammage makes the paper thicker, stiffer and better at carrying load, but box compression strength does not rise in direct proportion to grammage.

The European paper industry federation (CEPI) list of corrugated case materials treats bursting strength and short-span compression in the cross direction (SCT-CD) as the two most important mechanical figures of kraftliner. For brown kraftliner, CEPI sets a minimum burst index of 3.5 kPa·m²/g for paper under 250 g/m² and 3.0 for paper of 250 g/m² and above; the minimum SCT-CD index is 18.0 and 17.5 Nm/g respectively.

For moisture resistance, kraftliner should be sized to reduce water absorption; CEPI gives a typical 1-minute Cobb water absorption of about 25 to 45 g/m². When higher moisture resistance is needed, the paper can be coated or laminated with plastic film.

2. Types of Kraft liner and their pros and cons

Liner is classified by fibre source, because the fibre source decides strength, consistency and price. By the definition commonly used in the industry, kraftliner contains at least 80% virgin Kraft fibre and testliner contains less than 80%. In practice many liners blend both sources, and once the fibre is dry it is very hard to tell them apart by eye, so the manufacturer’s data sheet is the real basis.

2.1. Virgin Kraft liner (kraftliner)

Kraftliner is made mainly from virgin Kraft pulp, with long cellulose fibres, so it has high tensile strength, tear strength, bursting strength and compression strength at the same grammage. The paper is consistent, has few impurities and prints well.

It is used for boxes that need high compression strength: export boxes, heavy goods, tall stacking in warehouses or containers, and packaging that needs cleanliness such as boxes for dry food (check the food safety requirements of the destination market).

2.2. Recycled liner (testliner)

Testliner is made from recovered paper pulp such as old cartons and used paper. Its advantage is a lower price; according to the containerboard definitions of Robert Guide, in European and Asian markets testliner is usually about 20% cheaper than kraftliner. It is also a way of making use of recycled fibre.

The drawbacks are that the fibres get shorter with each recycling pass, so strength is lower at the same grammage, the paper can be more sensitive to moisture and can contain impurities. Testliner suits boxes that do not need very high stacking, domestic packaging, or the inner face of a box.

3. The role of Kraft liner in packaging quality control

Liner quality is the key factor that decides the load capacity of a carton, especially in logistics environments with stacking and vibration.

3.1. Edge crush resistance (ECT) and box compression strength (BCT)

Edge crush strength (ECT, Edge Crush Test) measures how well the board resists compression along its edge. ECT depends on grammage, liner quality and the stiffness of the fluting; virgin liner gives a higher ECT than recycled liner at the same grammage.

Box compression strength (BCT, Box Compression Test) measures the vertical force the whole box can withstand when stacked. The simplified McKee formula presented by Georgia Tech estimates BCT = 5.87 × ECT × √(Z × t), where Z is the box perimeter and t is the board thickness. In other words, a better liner raises ECT and BCT rises with it, but the gain also depends on box size and flute type.

3.2. In practice: cartons when blocking with dunnage bags

When inflated, a dunnage bag presses on the carton face; when the vehicle brakes or the ship rolls, this force rises further. A box with a good liner that stays dry holds its shape; a box with weak or damp liner dents at the pressed area, the void widens and the bag loses its pressing force.

In my experience, when advising on dunnage bags for cartons going by sea, I prefer boxes with kraftliner on the outer face, or liner with a moisture barrier such as PE coating, and I always inflate to the bag’s proper working pressure (0.2 bar for a Level 1 paper bag) so the box is not dented at the moment of inflation. How to read dunnage bag pressure and the other bag metrics is covered in the article on technical metrics.

3.3. How is liner different from dunnage bag shell paper?

Many people think paper dunnage bags are made from liner paper. In fact the shell of the ICD Vietnam Kraft paper bag is Kraft paper laminated with plastic film, about 65 gram by the ICD Vietnam construction diagram, much thinner and more flexible than carton liner (usually from 115 g/m²). Bag shell paper has to be flexible to fold, inflate and carry gas pressure, while liner has to be stiff to carry compression when stacked.

4. Choosing the best Kraft liner for the supply chain

To balance cost and safety, a business can combine liner types according to transport and storage requirements.

A general rule: use kraftliner first for the outer face, where impact, printing and moisture exposure occur. The inner face and the fluting can use recycled paper to lower cost if the required compression strength is still met. The specific grammage should be chosen by the BCT needed, calculated from weight and the number of stacked tiers, and then confirmed by testing sample boxes instead of choosing by feel.

When ordering paper or boxes, ask the supplier to state on the data sheet: liner type (kraftliner or testliner), grammage, burst index, SCT and Cobb. This is the only way to compare two quotes with the same grammage but a different fibre source.

Frequently asked questions about Kraft liner

What is the lowest grammage of Kraft liner?

According to PaperIndex, kraftliner on the market has a grammage of about 115 to 440 g/m², and shipping packaging commonly uses 115 to 200 g/m². The lower the grammage, the more ECT and BCT usually fall, so choose by the compression strength needed.

How do Kraft liner and Kraft fluting differ?

Liner is the flat outer and inner layer of the board, taking bursting, compression and abrasion and giving the printing surface. Fluting is the middle layer pressed into flutes, keeping the two liners apart, making the board stiffer and cushioning shock. Fluting usually uses recycled or semi-chemical fibre.

How do kraftliner and testliner differ?

By the commonly used definition, kraftliner contains at least 80% virgin Kraft fibre and testliner contains less. Kraftliner is stronger at the same grammage; testliner is cheaper, usually about 20% lower in Europe and Asia.

Can Kraft liner be used for direct food packaging?

Clean virgin kraftliner has few impurities and is often chosen for dry food boxes, but you need to check the food safety requirements of the destination market and the manufacturer’s certification. If moisture or grease resistance is needed, use liner with a suitable coating.

Is the shell of a paper dunnage bag made from Kraft liner?

No. The shell of the ICD Vietnam Kraft paper bag is Kraft paper laminated with plastic film at about 65 gram, thinner and more flexible than carton liner, because the bag has to fold and inflate.

Contact us for technical advice

References

• Properties of Kraft paper used as liner - TAPPI (Technical Association of the Pulp and Paper Industry).
• CEPI - European list of corrugated case materials (copy hosted by DS Smith) - definitions, burst, SCT-CD and Cobb figures for kraftliner.
• Robert Guide - Containerboard definitions - kraftliner, testliner.
• PaperIndex - Kraft paper grade guide - kraftliner grammage range.
• Georgia Tech - effect of moisture on carton compression strength.